1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a printer, using electrophotography, and a method of controlling the image forming apparatus.
2. Description of the Related Art
In general, in an electrophotographic image forming apparatus, a photosensitive surface of an image bearing member, such as a photosensitive member, is uniformly charged by an electrostatic charger, and an electrostatic latent image corresponding to image information is formed on the charged photosensitive surface by a latent image forming unit. Then, the electrostatic latent image is developed by a developing device using a developer and transferred onto a recording sheet by a transfer unit, whereby an image is formed.
In the image forming apparatus, the density and density gradation characteristics of an output image are sometimes different from those of an original image due to influence e.g. of a short-term change caused by a change in an environment where the apparatus is installed or an environmental change within the apparatus, or a long-term change caused by aging (degradation) of the photosensitive member or the developer.
Therefore, in the image forming apparatus, it is necessary to correct image forming conditions, as required, by taking into account the above-mentioned various changes, so as to make the density and density gradation characteristics of an output image equal to those of an original image.
In an image forming apparatus of the above-mentioned type, the toner density of two-component developer (the ratio of a toner weight (T) to the weight (D) of the total sum (developer) of a carrier and toner) is mentioned as one of very important factors required for stabilization of image quality. The toner density of developer decreases during development which consumes toner. For this reason, in the image forming apparatus, the toner density of developer or the density of a test image formed on a photosensitive member, an intermediate transfer member, or a recording sheet is detected using a density control device, i.e. a developer density control device or an image density control device. Then, the density control device replenishes a developing device with toner from a toner holder according to a change in the detected toner density of the developer or that of the pattern image. The toner density of the developer or the image density is held as constant as possible by this control, whereby an excellent image quality is maintained.
As an example of a method of controlling toner density, there has been proposed the developer reflection ATR (auto toner replenishment). A density control device using the developer reflection ATR optically detects the toner density of developer within a developing device by detecting an amount of reflected light from toner irradiated with light using a toner density sensor, and the amount of toner replenishment is controlled according to the result of the detection.
Further, as another example of the method of controlling toner density, there has been proposed the so-called patch detection ATR which detects the density of a pattern image. A density control device using the patch detection ATR forms an image density detection pattern image (patch image) for reference on an electrophotographic photosensitive member (photosensitive member). Then, the density of the pattern image is detected by a sensor, such as an image density sensor, disposed in facing relation to the photosensitive member, and the toner replenishment amount is controlled according to the result of the detection.
Furthermore, as a further example of the method of controlling toner density, there is proposed the so-called video count ATR. This method calculates the amount of toner to be consumed based on pixel-by-pixel output levels of a digital image signal from a video counter, and the amount of toner to be consumed is supplied for replenishment.
Each of the density control devices using the respective above-mentioned control methods controls the rotation or the like of a motor for driving a toner replenishment unit, whereby the amount of toner to be supplied for replenishment of developer within a developing device from a toner holder is controlled. Thus, the density control device holds the toner density of the developer or image density at a predetermined target density.
However, the above-described density control devices suffer from the following problems: First, in the case of the developer reflection ATR, the toner density of developer within a developing device is directly detected, which enables the toner density of the developer to be held constant. However, the frictional charge amount (triboelectric charge amount) of toner changes due to changes in the quality of a magnetic carrier in developer or environmental change, which causes a change in developing performance. For this reason, even if the toner density of the developer can be held constant, when the quality of the magnetic carrier or the environment changes, images are sometimes not formed with a desired density due to the fact that the toner charge amount does not become equal to a predetermined charge amount.
In the video count ATR, the density information of an original image is converted to a video count value, the amount of toner to be consumed is estimated based on the video count value, and toner corresponding in amount to a predicted change from an initial setting of the toner density of developer is supplied. For this reason, when the amount of actually consumed toner and the toner consumption amount estimated based on the video count value differ from each other, the difference occurs in the toner replenishment amount which should correspond to the amount of consumed toner. In such a case, the toner density of developer can deviate from its initial setting.
Further, in the case of the patch detection ATR, the control is performed by detecting the density of a patch image on a photosensitive member, so that image density can be maintained at a predetermined target value.
However, if the toner density is controlled by the patch detection ATR alone, there arises the following problems: If the amount of toner attached to the electrostatic latent image decreases due to a rise in frictional charge amount (triboelectric charge amount) under a low-humidity environmental condition, low patch image density is detected by the patch detection ATR, and therefore the control is performed such that toner replenishment is continued. When the toner/developer ratio is high, if the toner replenishment is carried out based on the result of patch detection ATR, the developing device is overfilled with toner by excessive toner replenishment, which causes overflow of developer or fogging.
On the other hand, if the amount of toner attached to the electrostatic latent image increases due to a fall in frictional charge amount (triboelectric charge amount) under a high-humidity environmental condition, a high patch image density is detected by the patch detection ATR, and therefore the control is performed such that the inhibition of toner replenishment is continued. When the toner/developer ratio is low, if the toner replenishment is inhibited based on the result of patch detection ATR, the amount of toner in the developing device decreases to reduce the amount of coating of developer on the developer bearing member, which can cause degradation of images.
To overcome the problems, the toner density is controlled to be as uniform as possible by the developer reflection ATR or the video count ATR and at the same time, the toner density is controlled based on the patch detection ATR such that an output image is formed which has a desired density (desired maximum density and desired gradation characteristics)
A conventional image forming apparatus performs image formation while restricting the control of the toner density of developer by the ATR control (see e.g. Japanese Patent Laid-Open Publication No. 2007-78896). In the conventional image forming apparatus, based on a toner density of developer detected by the developer reflection ATR and an image density of a patch formed by the patch detection ATR, it is determined whether or not the result of the patch detection ATR is to be reflected on the toner density control. Further, the toner density control is performed based on the results of the developer reflection ATR and the video count ATR without causing the result of the patch detection ATR to be reflected, and at the same time, the control of the image contrast potential for suppressing density variations of output images is performed. Here, the development contrast potential indicates the potential difference between the potential of a DC current component of the developing bias and the light area potential (image area potential) on the photosensitive member. In the conventional image forming apparatus, if the toner/developer ratio detected by the developer reflection ATR is equal to or larger than a predetermined value, and at the same time the image density detected by the patch detection ATR is not within a predetermined range and hence the image is darker than it should be, the development contrast potential is controlled to be increased. If the toner/developer ratio detected by the developer reflection ATR is lower than the predetermined value, and at the same time the image density detected by the patch detection ATR is not within a predetermined range and hence the image is lighter than it should be, the development contrast potential is controlled to be reduced. This makes it possible to adjust the image density to a desired value when the toner/developer ratio is not proper.
In other words, when the toner/developer ratio is at an upper or lower limit of its proper range, the conventional image apparatus performs the control such that a detection result of the variation in the density of an output image is fed back to the development contrast potential.